AC motors are used in a variety of applications, including vehicle applications such as traction control. The AC motors used in vehicle applications are typically controlled via a voltage source inverter. Discontinuous Pulse Width Modulation (DPWM) methods are commonly employed in inverter controllers to control the fundamental output voltage component of three-phase voltage source inverters. These three-phase voltage source inverters may in turn be used to control the phase currents of three-phase AC motors. DPWM methods reduce inverter losses in comparison with continuous Pulse Width Modulation (PWM) methods, such as sinusoidal or space vector modulation.
DPWM methods generally differ from continuous PWM methods in that DWPM methods use a single zero vector in a given switching cycle of the three phase voltage source inverter. Additionally, in DWPM methods, each switch in the three phase voltage source inverter is typically not switched, or clamped, for sixty degree (60°) segments of an electrical cycle. The location of the sixty degree (60°) clamped segment with respect to the output voltage of the three phase voltage source inverter and the load power factor generally determines the type of DPWM method and resulting PWM properties. For example, when the clamped segment is located such that losses are optimized for a thirty degree (30°) leading power factor load, the DPWM method is referred to as DPWM0. DPWM1 refers to when the clamped segment is located to optimize switching losses for a unity power factor load, and DPWM2 refers to when the clamped segment is located to optimize switching losses for a thirty degree (30°) lagging power factor load.
Most DPWM methods used with voltage source inverters are susceptible to voltage distortion due to practical limitations of the voltage source inverter, such as inverter lockout time, or dead time, and minimum pulse width constraints. These practical limitations are typically non-linear effects that manifest as finite and controllable minimum and maximum pulse widths. Either inverter switch, for a phase leg, of the voltage source inverter can be indefinitely held “ON” to create discrete values of pulse widths with duty cycles of zero and one, respectively. During some operating conditions, the commanded duty cycles for a particular phase leg have a pulse width between the minimum and maximum achievable pulse widths and the corresponding discrete values of zero and one. The non-linear effects produce unachievable regions (e.g., between the minimum and maximum achievable pulse widths and the corresponding discrete values of zero and one) that occur for each phase of the voltage source inverter.
Accordingly, it is desirable to provide a method for minimizing inverter switching losses when operating a three phase AC motor. In addition, it is desirable to provide a voltage source inverter having minimized switching losses for controlling an AC motor. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.